Long-term variability in occurrence frequencies of magnetic storms with sudden and gradual commencements

Abstract

Long-term variations in annual frequencies of occurrence of magnetic storms with sudden and gradual commencements were studied on the base of the data from IZMIRAN and Slutsk (Pavlovsk) magnetic observatories for the period 1878–2015. It was found that occurrences of strong and moderate magnetic storms with gradual commencements are characterized by a pronounced variability on the multidecadal time scale. Their wavelet spectra reveal strong periodicities of ~36 years (close to the climatic Brückner cycle), as well as less pronounced ~60-year and ~90-year ones throughout the entire time interval under study. The occurrences of strong, moderate and weak magnetic storms with sudden commencements are characterized by dominating ~11-year periodicities, whereas long-term variations are substantially weaker. The obtained results provide evidence for a different temporal evolution of local and global solar magnetic fields responsible for solar agents (CMEs and high-speed solar wind streams from coronal holes) contributing to the development of these types of magnetic storms. The obtained results allow suggesting a possible contribution of solar/geomagnetic activity to the formation of climatic oscillations on the multidecadal time scale.